The present invention generally relates to generator over-voltage protective circuits and, more specifically, to generator control unit circuits having a directly controlled switch for fast field discharge.
There are various known control circuits for dissipating the field energy of a field winding of a generator in an over-voltage condition. FIG. 3 shows a circuit 100 of known prior art for discharging the field current. A fast field discharge circuit 102 includes an additional switch 104 in series with a fly-wheeling diode 106. The additional switch 104 is normally kept closed and is opened upon detection of an over-voltage. An RC snubber is also used across the switch 104 to protect it from excessive inductive kick. An isolation switch 108 is used to address the effect of a shorting failure of main buck switch 110. Both the isolation switch 108 and the additional switch 104 of the fast field discharge circuit 102 are floating and require expensive isolated gate drivers with respective floating power supplies.
Referring to FIG. 4, U.S. Pat. No. 4,455,585 describes a control circuit 150 having an excitation winding L of an alternator connected in series with a controlled electronic switch 152, and an electronic regulator 154 connected to the electronic switch 152 for controlling the opening of electronic switch 152 when an output voltage of the generator exceeds a predetermined value. A regulator circuit 154 is fed by an output of the generator output and has an input IN connected to the intermediate terminal of a voltage divider consisting of two resistors R1 and R2 which are connected between the terminals 156 and 158. The control unit 150 further comprising a protective device connected in parallel to the excitation winding L and comprising a semiconductor circuit 160. An electronic switch 162 operated by an additional output US of the regulator 154 is in parallel with the semiconductor circuit 160 so that it is closed during the normal operation of the generator and opens when an over-voltage occurs on the generator output. The circuit is provided with more than one electronic power switch which increases the number of components constituting the control device, leading to complexity of the circuit and cost increase.
Referring to FIG. 5, U.S. Pat. No. 6,191,562 relates to a circuit configuration 200 for discharge of field energy. The circuit 200 includes a generator G with a field winding 202 characterized by its inductor L and its ohmic resistor R. The energy quenching of field winding 202 is controlled by a power transistor T which in normal operation, is operated with timed pulses from a clock control 204. The field winding 202 and power transistor T are connected in series to a battery voltage Ubatt and a power switch S in series with a diode D1 form a free-running circuit of the field winding 202. The circuit configuration 200 further includes a diode D2 and a zener diode DZ that are arranged between field winding 202 and power transistor T in such a way that, when the power switch S is opened, current can flow from field winding 202 via diode D2 and zener diode DZ to the control connection of power transistor T. When the battery is not completely cut off or a connection failure occurs anywhere in a power feeding cable, comparatively small surges of voltage are repeatedly applied to the power zener diode DZ irregularly within a short period. In this case, a large amount of heat accumulates due to the repetitive generation of heat, even though in comparatively small amounts. Therefore thermal damage becomes larger in this case than in the case where large surge of voltage generated when a rated load is cut off or disconnected. Moreover, the increased complexity of the circuit is undesirable.
As can be seen, there is a need for a simple and reliable control circuit having inexpensive power regulators that can quickly dissipate the field energy without causing any damage to equipment supplied by the generator. Such a control circuit would be safe and dissipate the field energy without generating considerable heat stress in the system. Further, such a needed circuit would associate a directly controlled regulator for convenience and would operate on the buck regulator shorting failure conditions.